The present invention relates to a significant need in the interferometer field. Relatively high resolution, relatively heavy (e.g., 75 to 80 lbs.) interferometers designed primarily for laboratory use generally operate in environments in which the interferometers are not mechanically coupled to vibrating equipment. There is a need for small (e.g., 5 lbs.) rugged interferometers which can be operated while mechanically coupled to vibrating apparatus without losing their optical efficiency because of such vibrations.
One use for such small interferometers is in portable units, where they are connected to a detector-cooling apparatus. Other uses include space vehicles, and generally any situation where the interferometers might be temporarily or permanently subjected to mechanical vibrations of nearby apparatus.
The primary field of interest of the present invention relates to short-stroke (e.g., 1 cm) inferferometers which can provide resolution of one-half wave number. Such small interferometers should have a reasonably large aperture, but be light, compact and rugged.
Ruggedness, in general, in this context is defined as imprerviousness to mechanical vibrations which would adversely affect the performance of the interferometer. Such adverse effects occur if any functioning parts of the interferometer resonate to the mechanical vibrations of extraneous apparatus. Coolers, fans, air compressors, other motor-containing equipment, etc., all are possible sources of interferometer-disabling vibrations.
Perception of the need addressed by the present invention came in large measure from analysis of the apparatus identified as the "XM21 alarm system". This system incorporates a detector mounted in a cooling apparatus, and a small interferometer supported on the cooling apparatus. In that interferometer structure, which is shown in FIG. 1, the moving mirror is moved by a torque motor, and is supported in a parallelogram arrangement having flex pivots connecting the parallelogram structure to the mirror carrier and to the stationary frame.
The XM21 design suffers from acute vibration sensitivity. It is too flimsy to allow a high gain, wide band mirror-drive loop to be incorporated with this interferometer. As a result, vibrations generated by the detector cooler cause significant mirror velocity variations. The XM21 program is an effort to build a highly sensitive system for remote detection of chemical agents. Its proposed characteristics are: (1) passive system; (2) all weather operation; (3) high probability of detection for all gas agents; and (4) low false alarm rate. To achieve these goals, the XM21 system includes an interferometer and a closed-cycle cooled HCT (mercury-cadmium-telluride) detector. Unfortunately, the cooler-generated vibration causes unacceptably large mirror velocity errors in the interferometer. Initial attempts to solve the problem were limited to vibration-isolating the interferometer from the cooler.
Susceptibility of the XM21 alarm system to vibration-caused interferometer problems led to the incorporation of elements which mechanically isolate the interferometer from the remainder of the system. While this constitutes an obvious "fix" for the vibration-caused interferometer problems, it constitutes, in effect, a submission to the problem, rather than a solution of it. Unfortunately, the use of isolating means creates the problem of relative displacement between the interferometer and the external elements with which it interacts, such as the radiation source, the sample (whatever is being analyzed), and the detector. Thus, undesired noise modulation is caused by the use of isolation as a means of avoiding vibration problems in the interferometer.
Since the mirror-drive loop is a closed servo loop, it necessarily includes mechanical elements in the overall loop. In the case of the XM21 design, the three mechanical pieces in the loop are the forward support arm, the flex pivots, and the moving mirror support assembly. The transfer function of these elements will have a strong influence on the stiffnes of the overall loop. The frequency response of the loop electronics must be tailored to keep the loop gain below one (1) at any resonance in the mechanical support structure. In other words, any phase shift in the mirror-drive loop must not reach 180.degree., because at that point the feedback control signals shift from negative feedback to positive feedback.
Because understanding the relatively complex background of the present invention is vital in appreciating its significance, a much more detailed analysis of the problem will now be set forth.
The efforts of suppliers of interferometer systems, in counteracting vibration problems, have generally used one of two concepts, each of which has complexity problems, and neither of which actually attacks the basic difficulties created by ambient vibrations.
One of the concepts, used by several suppliers, has been directed toward developing "tilt immune" optical systems, which incorporate complex and unwidely optics. They sacrifice the simplicity and low cost of the plane mirror optical system; and they to be large and awkward for a given level of performance.
The other concept, also used by several suppliers, has been directed toward automatic alignment systems, which incorporate complex serve systems. They use elaborate electronic systems to do a job which could be accomplished more effectively and less expensively with clever mechanical design and precision machining.
Even more importantly, those who have adopted either of the two concepts identified above have been lead seriously astray in their efforts to design interferometers which are able to function efficiently in spite of "hostile" environments. Their concentration has been primarily on the problem of "static misalignment", neglecting the more vexing problems of (a) "dynamic misalignment", and (b) the even more serious "mirror velocity errors".
Neither of the types of interferometers discussed above attacks the effects of ambient vibration on: (a) alignment, and (b) mirror velocity. Such interferometers require the user to isolate the interferometer from the ambient vibrations in order to function. Such isolation is never completely successful and carries its own hazards. Most often, isolation of the interferometer from a vibrating ambient also isolates it from a vibrating source, or sample. The relative motion between the source, or sample, and the interferometer induces unwanted modulation in the signal. Since this modulation creates a noise which is proportional to the signal level, it destroys the chief advantages of the interferometer.
The effects of ambient vibration on mirror alignment are obvious. Any vibration whose frequency is higher than the lowest frequency ressonance of the support structure will cause the mirrors to vibrate. Ambient vibrations which substantially correspond to the mirror support resonances will cause very large vibrations of the mirrors. These dynamic misalignments induce both amplitude and phase errors in the interferogram. Such errors also induce noise proportional to the signal.
The effects on mirror velocity are far more subtle and pernicious. This is due to the interaction between the structural resonance and the mirror drive control loop. As stated above, the mirror drive control loop must have a gain less than one at the lowest frequency resonance of the structure. To be precise, the gain of the loop times the "Q" factor of the structural resonance must be less than one. As a practical matter, to achieve this condition, the gain of the loop must be low to start with, and rolled off at a low enough frequency to drop below one, at a frequency significantly below the lowest frequency resonance. As a consequence, the mirror drive loop cannot compensate for ambient vibrations which are higher in frequency than the lowest frequency structural resonance. Therefore, ambient vibrations, which do not bother the dynamic alignment, can nevertheless have severe effects on the mirror velocity.
The problem discussed in the preceding paragraph is particularly difficult to solve because, generally, high loop gain is desirable, in order to compensate for ambient vibrations.
An additional problem which tended to obscure the effects of mirror velocity errors was the poor state of theoretical modeling of their effects. There is very little information in the literature on their effects and until recent work by Zachor ("Drive Nonlinearities; Their Effects in Fourier Spectroscopy"--Applied Optics 1977--P. 1412), such information was wrong. The Aspen Proceedings, which are generally taken to be the best source of information for interferometer designers, have only one mention of mirror velocity errors, and that one was grossly inaccurate. This is in contrast to the fairly widespread understanding of the effects of mirror tilts.